Filter or filter-element for modified electro-dialysis (MED) purposes

ABSTRACT

Filter or filter-element designated for Modified Electro-Dialysis (MED) purposes characterized in that the filter or filter-element comprises a porous, ceramic, mainly uniform material with functional, preferably ion selective groups grafted onto the inner, porous surface of the ceramic body. The outer surface of the filter or filter-element may be completely or partly covered by layers of porous, ceramic membranes with a pore size of less than 1 μm and thickness less than 1 mm, and/or anion, cation or bipolar groups or membranes. The thickness of the filter-element is larger than 1 mm and has pores of size larger than 1 μm. The invention also relates to a method for the manufacture of such a filter either continuously by tape-casting, extruding, rolling or calendaring or single-bodied by casting, pressing or forging, of a paste containing a non-conductive, ceramic material. The body is then sintered and finally grafted with functional, preferably ion selective groups for one or more specific ions or groups of ions, onto the inner, porous surface of the ceramic body. Use of the filter or filter-element for filtering ions or complexes of heavy or precious metals from water is also claimed.

This application is a filing under 35 USC 371 of PCT/NO01/0282 filedJul. 3, 2001.

BACKGROUND OF THE INVENTION

The invention relates to a filter or filter-element suited for ModifiedElectro-Dialysis purposes, particularly for purification ordemineralizing of liquids with respect to impurities in the form of ionsor ionic complexes of heavy metals or noble metals. The invention alsoincludes a method for manufacturing such a filter or filter-element. Theinvention further relates to the use of such a filter or filter-element.

BACKGROUND

Heavy metals represent a problematic waste for several types ofindustries with concentrations that are often unacceptably high. Thetoxicity of many of these elements is very high and the tendency tocontaminate the environment is a great concern.

Heavy metals generally represent a special environmental problem sincethe elements cannot be destroyed but must be isolated or reduced totheir original, elementary state.

Historically heavy metal containing waste has mainly been treatedchemically resulting in a hydroxyl- or sulphide-containing sludge thatwould have to be deposited. Such “end-of-pipe” solutions require largeamounts of water aid chemicals, and hence create new environmentalproblems. Amongst the larger producers of this type of wastes aremineral processing industries and metal processing (galvanizing,plating, coating) industries.

Such waste deposits represent an increasingly growing problem fortoday's society; hence authorities in most industrialized countries haveimposed restrictions and legal regulations for such waste emissions anddepositions. European countries, headed by the European Union, havelately introduced new emission limits for heavy metals in industrialwastewater. These PARCOM limits—with corresponding US limits—will makeup the future emission limits for industrial heavy metal emissions.

Due to increasing costs related to the deposit of industrial wastesludge, there is a growing interest in the industry for finding newsolutions for recuperation and recycling of heavy meals in industrialprocesses. This will reduce costs for both waste handling and depositand for the metal/metal complexes of the process. In addition the volumeof deposits is reduced

A corresponding situation exists for precious metals. Due to the higheconomical value of these metals it is found attractive to extract theminor amounts of metals that are found in the processing and rinsingwaters.

Also for the ultra pure waters used for products and processing invarious industries (e.g. semiconductor industry, pharmaceuticalindustry, other medical and health care industries and services), ionsneed to be removed from the process water streams.

For industrial wastewater “end-of-pipe” solutions are still thepredominant. These solutions have, amongst others, the followingdisadvantages:

-   high water consumption,-   high consumption of chemicals,-   loss of costly metals and other chemical ingredients,-   production of large amounts of environmentally toxic sludge,-   costly transport and disposal of the sludge.

Alternative methods for purification of metal ion containing wastewaterare: evaporation, reverse osmosis (RO), electrodialysis (ED), ionexchange (IE) and electrolysis. These are all established methods, butnone of them are able to meet the PARCOM-limits alone.

Modified Electrodialysis (MED) is a combination of ED and IE. The methodutilizes, in principle, the equipment from electrodialysis, with analternating arrangement of anion and cation membranes. The ion exchangeris confined between a specific set of these membranes and may beresponsible for the selectivity of the method and the ability to treatvery diluted liquids. This is described in detail below, with referenceto FIG. 1.

MED is a new method for a continuous and selective recuperation andremoval of metal ions from wastewater, which is capable of meeting thePARCOM-limits.

A similar method is used for purification of water to be used as processwater with extreme requirements to purity and lack of ions of any kind(e.g.: semiconductor industry, pharmaceutical industry, other medicaland heal care industries and services). This non-selective process isnamed in the literature as EDI (Electro DeIonization) or CEDI(Continuous EDI).

Historically the EDI/CEDI concept is relatively old. The first reportsand patents date back to the mid 1950's when the method was developed inorder to purify wastewater from nuclear plants of radioactive elements.The first patents are registered by P. Kollsman (U.S. Pat. No.2,815,320), R. G. Pearson (U.S. Pat. No. 2,794,777), T. R. E. Kressman(U.S. Pat. No. 2,923,674) and E. J. Parsi (U.S. Pat. No. 3,149,061).

In the 1970's the EDI/CEDI-process was reinvented with the aim toproduce ultra-pure water and to purify potable water. In the middle ofthe 1980's the first commercial CEDI units were launched into themarket, headed by Millipore, cf. U.S. Pat. No. 4,632,745.

Today's CEDI equipment utilizes either mixed-bed or single-bed ionexchangers confined by a combination of anion and/or cation membranes,see e.g.: WO 98/11987. Also the utilization of bipolar membranes isdocumented, see U.S. Pat. No. 4,871,431 and U.S. Pat. No. 4,969,983.

Common to all the different CEDI concepts is that the active cells areconstructed by a multiplicity of separate components, which introduces amix of organic and inorganic elements of varying strength, wearproperties and stability, see e.g.: WO 95/29005. Important parametersfor cell construction—in addition to low electrical resistivity—aremechanical, thermal, and chemical stability, which should all be high.Consequently the construction of compartments for liquid flows (both thediluted and concentrated flows) is important. Generally supports and/orspacers are used in order to meet the very narrow geometricalspecifications necessary to ensure homogeneous flow patterns and lowelectrical resistance. This is described in several patents, both withrespect to systems solution, see e.g.: WO 97/25147, EP 853,972, and U.S.Pat. No. 5,681,438, and with respect to supports and spacers, see e.g.:EP 645,176 and U.S. Pat. No. 4,804,451.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a uniform, mechanicallystrong, and mechanically, thermally and chemically stable filter orfilter element suitable for removal of ions or ion complexes of heavy orprecious metals from liquids.

It is also an object of the invention to provide a filter orfilter-element where the flow pattern for the liquid is sufficientlyhomogeneous and open (permeable).

It is a further object of the invention to provide a method for themanufacture of such a filter or filter-element, where the productioncost is within acceptable and competitive limits.

The above mentioned objects are achieved by the filter or filter-elementaccording to the invention, which constitutes a new filter-element toMED-systems (including EDI/CEDI) acting as a homogeneous and singlereplacement for the complete diluting, concentrate, and/or electrodecompartments, including the combination of the specific ion exchangerwith container, support, spacer, and anion/cation membranes.

Advanced ceramic products are today manufactured by first making a doughor paste consisting of: 40–60% ceramic powder, 2–10% binder, 2–10%softener, 1–2% dispersant, and 40–60% solvent. This dough or paste canbe formed plastically into products or bodies (“green-bodies”¹) eithercontinuously by tape-casting, extruding, or calendering or single-bodiedby casting, pressing or forging preferably such that the geometry andshape is accurately defined. Thereafter, the “green-bodies” are sinteredor fired at high temperatures. During this sintering process all organiccomponents disintegrate, leaving only a fully ceramic finished product.

Filter-elements manufactured by this method may have an arbitrarygeometry, varying from highly regular circles, ellipses, squares,rectangles etc. to highly irregular free forms.

BRIEF DESCRIPTIONS OF THE DRAWINGS

In the following the invention is described in detail, includingexamples with reference to the enclosed figures, where:

FIG. 1 shows a principal view of a typical layout of a ModifiedElectro-Dialysis (MED) system according to known technology,

FIG. 2 shows a sectional view of a filter-element according to theinvention,

FIG. 3 shows a sectional view of a filter according to the invention,including the filter-element of FIG. 2 covered by thin anion/cationmembranes (if necessary including: thin, ceramic, porous membrane layerswith ion-selective groups) on two of the surfaces of the filter-element,and

FIG. 4 shows a sectional view of a filter comprising one type of innerdrainage channels.

FIG. 1 shows the layout of a Modified Electro-Dialysis (MED) system,which is a technology inherited from Electro-Dialysis (ED), whereinalternating anion and cation membranes constitute compartments orchambers for different liquid flows. In FIG. 1 these compartments aremarked C for concentrate, D for diluting (or clean liquid) and E forelectrode. Further, 1 indicates the feed flow (or diluted) in, 2 dilutedout, 3 concentrated in, 4 concentrated out, whereas 5 and 6 are theelectrodes. The feed flow to be cleaned (diluted) 1, is fed into thediluting compartments D. During passage through the dilutingcompartments D, the electric field, ∈, will conduct the charged anionsand cations of the feed flow in opposite directions out of the dilutingcompartments through the anion and cation membranes respectively. Theanion membranes prevents cations from moving from the concentratecompartments into the neighbouring diluting compartments, and similarlythe cation membrane prevents the anions from moving from the concentratecompartments into the neighbouring diluting compartments. This is how EDfunctions without the use of an ion exchanger. For very lowconcentrations of impurities (ions) the function and efficiency of theED process is strongly reduced. This is due to the low conductivity ofthe liquid at low ion concentrations. In order to resolve this problemthe MED technology introduces ion exchangers in the dilutingcompartments D, alternatively also in the concentrate compartments C.The ion exchanger will then absorb/extract the available metal ions,which will increase the charge density in the diluting (and concentrate)compartments and, assuming that the absorbed ions are sufficientlymobile, the electric field will still effectively be able to conduct thecharged ions out of the diluting compartments through the anion andcation membranes. Without the ion exchanger the efficiency of theprocess will be strongly reduced and the costs and the energyconsumption will strongly increase.

FIG. 2 shows a sectional view of the filter-element of this invention,showing a homogeneous core (k) that constitutes the filter-element inthe form of a substrate made of porous, ceramic, non-conducting materialwith large pores. The preferred size for these pores is at least 1 μm inorder to reduce the flow restriction for the liquid flows. A representsthe feed flow (e,g, diluted), and B₁ and B₂ are the flows of anions andcations being conducted out of the diluting compartment by theelectrical field, ∈. This single filter-element constitutes thestructure of the diluting compartment, the concentrate compartment andthe electrode compartment respectively, and ensures the mechanical andchemical properties. The filter-element has the similar functionalproperties as the ion exchanger used in conventional CEDI systems. Thisis achieved by grafting the preferred functional groups onto thecomplete inner surface of the porous, ceramic filter-element. Thegrafted groups may be selective to specific metal ions or not Thefilter-element described in FIG. 2 will however not confine the liquidflows but be open to leakage through the outer surfaces of the element.Hence, this element can only exceptionally act as a complete filter.

A typical thickness of the filter-element is between 1 and 10 mm,depending on the mechanical and functional demands on the element. Formixed-bed (both anion and cation groups grafted on the same element)applications the thickness must be limited due to transport properties.However for single-bed (only one type of active groups grafted on thesame element) applications the thickness may preferably be high in orderto increase the capacity and reduce the flow speed.

In order to “close off” one or more of the outer surfaces of thefilter-element of FIG. 2, membranes of anionic, cationic, or bipolarnature may be applied on these surfaces as described below. In suchcases a reduced pore size close to the outer surface of the element maybe preferred. This can be achieved by applying one or more thin ceramicmembrane layer(s) with the desired pore size. The methods for applyingthese layers may be: tape-casting, spraying, slip casting,screen-printing, gel-casting, or sol-gel coating. After drying theapplied layers are sintered at high temperature, yielding porous fullyceramic membrane layers.

The selected, functional (e.g.: ion-selective) molecular groups are thengrafted onto the complete, internal surface of the filter-element (withor without the outer ceramic membrane layers). The choice of thefunctional groups depends upon the element(s) to be removed from theliquid flow. Two common active, non-selective groups are complexes ofsulphonate and ammonium. However the supply of commercially available(low and high selectivity, strong and weak) groups and complexes oforganic and inorganic nature is large, and all such groups areapplicable in principle. Depending upon the actual structure of theactive group, these can be grafted either directly onto the innersurface of the filter-element by chemical, physical, or physio-chemicalmethods or indirectly with the aid of coupling reagents. These couplingreagents are organic or inorganic molecular groups preferably containingsilicon, titanium, phosphorous, boron, sulphur or nitrogen, and may bee.g. silanes, titanates, phosphates or others. The purpose of thecoupling reagent is to create a tight and stable binding to the innersurface of the ceramic filter-element. In special cases also radiation,e.g.: UV-, X-ray, γ-, or elementary particle-radiation, may be appliedin order to improve the binding.

When the filter-element material is alumina (Al₂O₃) this material isknown to have elementary OH-groups attached to the surface:—Al—OH.

Sometimes the alumina surface has to be activated. The purpose of thisactivation is to create the maximum number of OH-groups on the surface.

If the coupling reagent is a silane (R₍₁₎—Si—R₍₂₎) one of the groups(R₍₁₎) of the silane will react with one or more OH-groups on thealumina surface to e.g. H_(n)R₍₁₎ leaving in principle the followingbinding towards the alumina surface:—Al—O—Si—R₍₂₎

The other silane-group (R₍₂₎) can then be utilized as a coupling reagenttowards the active group, e.g. iminodiacetic acid (IDA), where X is areaction product:

This grafting of the active, functional groups on the entire innersurface of the filter-element can be done with a sufficient density offunctional groups (in the order of 1 meq/ml) compared with most of theconventional ion exchanger resins. The method of application of thegroups can be either deposit from gas phase, liquid phase, or a solidstate reaction.

FIG. 3 shows a cut through the filter-element according to theinvention, which consists of a homogeneous core (k) that constitutes thefilter-element in the form of a substrate made of porous, ceramic,non-conducting material with large pores, with thin layers (1) on two ofthe outer sides of the element consisting of porous, ceramic membraneswith fine pores. On the entire inner surface of this filter orfilter-element is grafted active functional chemical groups. In theouter layers (1) may also be incorporated anion, cation or bipolarmembranes. This can be achieved either by grafting of single groups asdescribed above, or by applying monomeric groups that can polymerize onthe surface. In special cases radiation, e.g.: UV-, X-ray, γ-, orelementary particle-radiation, may be used in order to complete thepolymerization. With a suitable pore size distribution for the outer,ceramic membrane layers, these grafted or applied groups will be able toclose off the pores and behave as dense anion, cation or bipolarmembranes, The preferred pore size of the outer, ceramic membrane layer(1) is less 1 μm, so that the applied anion, cation or bipolar membranesshould not penetrate too deep into the structure, thereby not formingtight membranes. In this way the filter or filter-element according tothe invention will replace the whole structure of the diluting,concentrate and/or the electrode compartment in a conventional MED(EDI/CEDI) system with one single, ceramic, functional filter.

The filter-element (k) and the membrane layers (1) can in principle bemanufactured by all types of ceramic material. However, based uponavailability, price and their properties the preferred ceramic materialsare Al₂O₃, TiO₂, ZrO₂, SiO₂, or combinations, mixtures or phases derivedfrom these.

FIG. 4 shows a cut through a filter with one type of internal drainagechannels. The introduction of different drainage channels may improvethe flow passage through the filter-element, and hence reduce the flowresistive for the liquid passing through the element. This can beachieved during the manufacturing process when the filter-elements arein the “green” state by purely mechanical means or by inserts organictemplates into the “green-bodies” that will disintegrate during thesintering cycle. The use of such drainage channels represents in manycases the preferred embodiment of the invention. If the application sodemands, the drainage channels may be made so large that thefilter-element may be split into two or more separate parts.

Although the invention is exemplified by means of references to theenclosed drawings, it is to be understood that the invention can bemodified in may different manners without departing from the generalscope of the invention. The invention is only limited by the claims.

For example, membrane layers (1) are shown only on two sides of thefilter-element in the illustrations while some applications may demandmembrane layers on three or four sides, or simply one side. For otherapplications the membrane layer may only constitute a ceramic, porousmembrane without the anion, cation, or bipolar membrane embedded.

There are also applications where these layers are not necessary, inwhich case the filter-element (k) constitutes the complete filter.

Other inorganic or ceramic ion-selective membranes have been documentedand patented. These are however all thin sheet membranes constitutingcation membranes, see e.g.: Ikeshoji (JP 1-47403) and Oya (JP 4-135645),or composite (supported) membranes, see e.g.: Bray (WO 96/10453),Kasbiwada (U.S. Pat. No. 5,087,345), Horie (JP 3-232521), and Hying (NO2000 0437). These membranes can act as improved anion and cationmembranes in conventional ED and MED systems, but they cannot replacethe whole thick sheet, multifunctional, uniform, ceramic filter orfilter-element of the present invention.

Most practical filter systems will consist of multiple single filters orfilter-elements stacked in line as indicated in FIG. 1. The present,functional filter can then either replace only the diluting compartment,or both the diluting and the concentrate compartments, and if necessaryalso the electrode compartment. Usually it will be convenient to mountthe filters in a holder or cassette of some kind, in order to keep theflow paths closed and leakage free, and also to prevent the filter orfilter-element from becoming exposed to unwanted, external strains orinteractions.

It is also possible to use filters with a shape that deviates from thoseshown with rectangular geometry and constant thickness, even if theseand circular filters constitute the most practical geometries, both withrespect to their manufacture and use.

The most pronounced advantages with the new ceramic filter according tothe present invention are that:

-   i) it constitutes a well-defined diluting/concentrate/electrode    compartment with geometrically stable properties that will not be    changed or restructured under mechanical, electrical or chemical    impact,-   ii) it can host a high density of active, functional groups in the    core of the filter,-   iii) it functions as a support (and spacer) for the outer anion,    cation and/or bipolar membranes,-   iv) it exhibits good bonding properties between the core element and    the outer anion, cation and/or bipolar membranes, and-   v) the variation and combination possibilities are large.

1. An apparatus for Modified Electro-Dialysis (MED), comprising: aplurality of adjacently arranged Modified Electro-Dialysis (MED)filter-elements, with alternating diluting elements and concentratingelements separated by anionic and cationic membranes; each of thediluting elements including an inlet for liquid containing impurities tobe cleaned, and an outlet for cleaned liquid, liquid flow within thediluting elements taking place from the inlet to the outlet; each of theconcentrating elements including an inlet for clean liquid and an outletfor liquid containing impurities, liquid flow within the concentratingelements taking place from the inlet to the outlet; and means forproducing an electric field for conducting anions and cations inopposite directions out of the diluting elements into the concentratingelements, wherein at least one of the diluting elements is a ModifiedElectro-Dialysis filter element comprising a sintered porous ceramicmaterial body having a substantially uniform structure with functionalgroups grafted onto inner, porous surfaces of the ceramic body. 2.Apparatus according to claim 1, wherein at least one of theconcentrating elements is a Modified Electro-Dialysis filter elementcomprising a sintered porous ceramic material having a substantiallyuniform structure with functional groups grafted onto inner, poroussurfaces of the ceramic body.
 3. Apparatus according to claim 1, whereinthe plurality of Modified ElectroDialysis filter elements are arrangedwith parallel axes, and the liquid flow from the inlet and outlet ofeach of the filter elements takes place parallel to the axes. 4.Apparatus according to claim 3, wherein the electric field is appliedperpendicular to the direction of liquid flow.
 5. Method for removal ofheavy metal ions from a liquid, comprising passing the liquid through afirst Modified Electro-dialysis filter-element constructed and arrangedfor liquid flow between an inlet and an outlet, and arranged adjacent toa second Modified Electro-dialysis filter-element through which cleanliquid is passed between an inlet and an outlet, and applying anelectric field to the liquid flow to cause the heavy metal ions to movefrom the first Modified Electro-dialysis filter-element, into the secondModified Electro-dialysis filter-element, wherein at least the oneModified Electro-dialysis filter-element comprises a sintered porousceramic material body having a substantially uniform structure withfunctional groups grafted onto inner, porous surfaces of the ceramicbody.
 6. Method according to claim 5, wherein the second ModifiedElectrodialysis filter-element comprises a sintered porous ceramicmaterial body having a substantially uniform structure with functionalgroups grafted onto inner, porous surfaces of the ceramic body. 7.Method according to claim 5, wherein the liquid flow takes place in adirection parallel to a longitudinal axis of each said ModifiedElectro-dialysis filter-element.
 8. Method according to claim 7, whereinthe electric field is applied in a direction perpendicular to saidlongitudinal axis.